Plasma generator



Sept. 28, 1965 R. M. PATRICK PLASMA GENERATOR 4 Sheets-Sheet l Filed March 29, 1961 v K S 152m.. m R. :v d R 0 N m m T R P N O Nm, .o. m T fl., s M. m WQ m m l a m g 3 A N H o m m. R M M Y W B zjoou Sept. 28, 1965 R. M. PATRICK 3,209,189

PLASMA GENERATOR Filed March 29, 1961 4 Sheets-Sheet 2 COOlgANT l O 5 IO l5 TIME IN SECONDS F E soo 1 con. P- 5 40o 0N [L E q 200 ARC 0N- o 5 |o l5 Tumi; IN sEcoNos E RICHARD M. PATR|cK INVENToR.

BYZM/ WMM WMM 5 9M ATTOR NEYS Sept. 28, 1965 R. M. PATRICK PLASMA GENERATOR 4 Sheets-Sheet 3 Filed March 29, 1961 Sept. 28, 1965 R. M. PATRlcK 3,209,189

PLASMA GENERATOR Filed March 29, 1961 4 Sheets-Sheet 4 COOLANT 6 MAGNET A r3' Q :wis: a s eb] 6 j Low VOLTAGE 57 D.c. SUPPLY (4 D.c. SUPPLY 32 COOLANT PROPELLANT 1 4 6| M/(xGNET RICHARD M. PATRICK INVENTOR.

www, J9, /Pwelj ATTORNEYS United States Patent O 3,209,189 PLASMA GENERATOR Richard M. Patrick, Winchester, Mass., assignor to Avco Corporation, Cincinnati, Ohio, a corporation of Delaware Filed Mar. 29, 1961, Ser. No. 99,207 12 Claims. (Cl. 313--63) The present invention is directed to plasma generators and more particularly to continuous plasma accelerators wherein the plasma is electromagnetically accelerated.

In order to clarify the description of the present invention it is of interest to note a definition of plasma, .particularly as here employed. Plasma is taken to mean an ionized gas composed of atomic or molecular particles, all or some of which have one or more orbital electrons removed therefrom and thus constituting ions together with a sufficient number of relatively free electrons to counterbalance the electrical charge of the ions whereby the resultant plasma is substantially electrically neutral, or in other words, is space-charge neutral.

Thus, plasma may be utilized in charged particle accelerators, pressure gauges, and other electrical devices wherein magnetic and electric fields are employed to operate upon charged particles such as ions to the end of observing the action of the charged particle under varying conditions of interest or producing particular end results with the charged particles. Although many wellknown ion sources employ an arc heated plasma as an ion reservoir from which ions are extracted, recent developments have identified applicability of plasma for purposes other than in the production of ions such as, for example, in rocket propulsion, studies of high energy air streams on air frames, and studies of blast effects. They are also useful as a neutral source of ions which may be heated and compressed to furnish a fusion reaction and thus may be particularly useful with controlled thermonuclear reactors.

In Patent Number 2,892,114, issued to W. D. Kilpatrick on lune 23, 1959, there is disclosed a plasma generator that is believed representative of prior art plasma generators capable of producing a continuous beam or jet of plasma as distinguished from a pulsed beam. The Kilpatrick device is of the single cylinder type wherein the cylinder is an anode and a flat plate disposed transverse to the longitudinal axis of the anode and outside of the cylinder but adjacent one end thereof is a cathode. The anode and cathode are connected `across a direct current, high voltage source and are disposed within a magnetic field.

In patent application Serial Number 752,309, filed July 3, 1958, of which applicant is a co-inventor, there is described a magnetic annular shock tube (hereinafter referred to as MAST) that is believed representative of prior art plasma generators or accelerators principally intended for producing a pulsed beam or jet of plasma as distinguished from a continuous beam `of plasma.

The MAST device utilizes a coaxial geometry with the plasma in an annular space where free motion in the azimuthal direction is allowed for the .plasma velocity and currents. A sufficiently high value of plasma electrical conductivity is required to prevent diffusion of the plasma and magnetic field to insure the successful operation of the MAST. A very high speed shock is used to create the conducting plasma. The flow is unsteady due to the passage of this shock, and the body forces which create the shock arise from the interaction of intense radial currents in the Iannulus and the azimuthal magnetic eld due to these same radial currents. The flow in such a device is created by the flow of the shock-heated plasma and necessarily, since this plasma is due to a shock wave, the flow is unsteady, i.e., constant for a very short period of time where the total duration of the flow is comparable to the time required for the shock to pass through the whole device such as, for example, a few microseconds for a small laboratory device.

Devices of the type described immediately hereinabove operate on a pulsed basis and utilize a plasma that is sufficiently conductive to prevent the diffusion of magnetic lines of force through the plasma in the time required for the plasma to pass through the device. The present invention, on the other hand, while it may utilize a coaxial geometry, operates on a continuous basis and the plasma conductivity is such that the magnetic eld can diffuse through the plasma in the time required for the plasma to pass through the device. Thus, ow is steady and is not due to a shock.

Briefly described, one embodiment of the present invention utilizes an annular geometry where the length of the lannulus is greater than the annular spacing formed by two concentric cylinders. A portion of the outer cylinder may include an outer ring electrode, and `a portion of the inner cylinder includes an inner ring electrode oppositely disposed from and surrounded by the outer ring electrode. A radial electric field is established across the ring electrodes by means of a direct current supply which supplies the dissipative power for a discharge between the ring electrodes. A magnetic coil, supplied from a source -of direct current, concentric with and surrounding the system .provides an axial magnetic field having weak radial and strong axial components throughout the device. The magnetic coil is preferably adapted to provide a slightly less intense magnetic field in the vicinity of the electrodes to stabilize the axial position of the discharge between the ring electrodes. A propellent gas is continuously supplied to`- one end of the annulus. An expansion nozzle, also surrounded by the magnetic coil, may be connected to the end of the system remote from the end at which the propellent gas in introduced.

Whatever the ultimate application of the present invention, certain substantial advantages are to be found in connection therewith and resulting from the 'advantageous plasma production. Contrary to conventional coaxial devices, the present invention is capable of producing a continuous beam of plasma as distinguished from a pulsed beam. Additionally, the present invention is particularly directed to the production in a plasma of directed energy in the azimuthal direction at a rate which is independent of the time required for the plasma to flow through the device, and which can be converted to directed energy in the axial direction in the expansion nozzle. Also, contrary to prior art devices, the ratio of the azimuthal to the axial velocity of the plasma can be readily controlled by variation of the operating parameters of the device.

The present invention is characterized by free motion of the plasma and currents in the plasma in the azimuthal direction, conversion of the azimuthal motion of the plasma to axial motion, minimum electrode heating, a high effective impedance across the discharge, and control of the plasma properties to provide substantially any one electron volt) with a ow velocity which correspondsv to a directed energy of 'several electrons volts.

It is an object of the present invention to provide means for producing continuous plasma jets.

It is another object of the present invention to provide means for heating and accelerating an ionized gas or plasma wherein the azimuthal Idistance over which the plasma moves during acceleration is independent of the distance it moves in the axial or iiow direction.

It is another object of the present invention to provide mean-s for increasing the effective impedance of the discharge through the use of an axial magnetic field and closed Hall currents in the azimuthal direction.

It is another object of the present invention to provide a plasma accelerator wherein plasma conditions are such that the electrical conductivity of the plasma is not sufiiciently high to prevent diffusion of magnetic field lines through the plasma.

It is another object of the present invention to provide a plasma accelerator for producing directed energy at a rate which is independent of the ow time of the plasma based on exhaust velocity and channel length.

A further object of the present invention is the provision of an improved and more efficient plasma accelerator that is continuous in operation and that utilizes a discharge having a high impedance and that is stabilized to prevent current concentration at one spot, thus reducing heating of the electrode surfaces.

A still further object of the present invention is the provision of a continuous plasma accelerator having a coaxial geometry which results in free motion of the plasma and currents in the plasma in the azimuthal direction.

Another object of the present invention is the provision of a continuous plasma accelerator characterized by the production of directed energy in the azimuthal direction at a rate which is independent of the time required for the plasma to How through the device, which azimuthal energy is converted to thrust energy in the expansion nozzle and which can be controlled by variation of the operating parameters of the accelerator.

A still further object of the present invention is the provision of a continuous plasma accelerator wherein the plasma properties can be controlled to provide substantially any ratio of the directed to thermal energy of the plasma, i.e., a hot plasma (at a temperature corresponding to several electron volts) having a high or low velocity or a cold plasma (at a temperature corresponding to less than one electron volt) having a high or low velocity.

The novel features that are considered characteristic of the present invention are set forth in the appended claims; the invention itself, however, both as to its organization and method of operation, together with additional objects and advantages thereof, will best be understood from the following description of specific embodiments when read in conjunction with the accompanying drawings, in which:

FIGURE 1 is a diagrammatic, longitudinal, sectional view of a preferred embodiment of the present invention, including electrical circuits connected to separate parts thereof;

FIGURE 2 is a sectional view taken on line 2-2 of FIGURE 1;

FIGURE 3 is a diagrammatic, longitudinal, sectional view of a second embodiment of the present invention similar to that illustrated in FIGURE 1;

FIGURE 4 is a graphic representation of magnetic field strength in the acceleration space;

FIGURES 5 and 6 are graphic representations of respectively discharge voltage and current in the present invention with and without the presen-ce of a magnetic field in the acceleration space;

FIGURE 7 is a fragmentary, diagrammatic, longitudinal, sectional view of another embodiment of the present invention; and

FIGURE 8 is a fragmentary, diagrammatic, longitudinal, sectional view of still another embodiment of the present invention.

Considering now the present invention in some detail and referring first to FIGURE 1 of the drawings, there will be seen to be included therein an inner electrically conductive, nonmagnetic, cylindrical member 11 concentrically positioned within an outer electrically conductive,

nonmagnetic, annular member l2 which `define a propellent acceleration space 13 having an open ended divergent end portion 14, a closed ended constant area end portion 1S, and Va constant area middle portion 16 interconnecting the end portions 14 and 15. When utilized as an accelerator the end portion 14 communicates with an evacuated space (not shown). The inner cylindrical member 11 and outer annular member 12 lare advantageously formed of copper to provide structural rigidity and to facilitate heat transfer from the acceleration space 13 to respectively the coolant jacket 17 surrounding annular member 12 and coolant passage 18 in cylindrical member member 11. A coolant such as water is passed through the coolant jacket 17 and cool-ant passage 18 to maintain the members 11 and 12 at the desired operating temperature. A cylindrical inner electrode 21 is embedded in the outer surface 22 of the inner member 11 and is concentric with and oppositely disposed from an outer electrode 23 embedded in the inner surface 24 of the outer member 12. Electrode 23 may be omitted if desired and when `such is the case the annular member 12 in addition to defining the acceleration space functions as the outer electrode for the arc discharge. Electrode 21 is in electrical contact with member 11 and electrode 23 is in electrical contact with member 12. Whereas the members 11 and 12 are preferably formed of an essentially nonemissive material, the electrodes 21 and 23 are formed of an emissive material such as, for example, tungsten, thoriated tungsten, and the like to provide optimum discharge characteristics. Inner cylindrical member 11 is electrically insulated from the outer annular member 12 by means of insulation 25 and is supported as a cantilever at its one end which passes through the end wall 26 of the annular member 12. The inner surface 24 of the outer member 12 and the outer surface 22 of the inner member 11 are each covered with respectively a thin layer of heat resistant insulating material 27 and 28 such as, for example, quartz or a ceramic. As will be evident hereinafter, this insulation may be omitted if desired.

Beginning at about the point of divergence in the outer annular member 12, the unsupported end portion 29 of the inner member 11 is tapered to provide a smooth transition from the constant area portion 16 to the divergent portion 14. A magnet coil 31, supplied with direct current from a suitable source 32, encloses the outer annular member 12 from a point at least intermediate its end wall 26 and electrodes 21 and 23 to the open end ofl the outer annular member 12. The magnet coil 31 may be formed in conventional manner to supply lines of forces, indicated by the arrows B, that pass through the acceleration space and that are everywhere substantially parallel to the inner surface 24 of the outer member 12, i. e., the magnetic field within the acceleration space 13 is shaped to prevent (as shown in FIGURE 1) the connection of member 11 with member 12 by lines of magnetic force, particularly downstream of the working section of member 11, such as, for example, electrode 21.

Since the middle portion 16 of the outer member 12 is provided with a constant area cross section and a divergent end portion 14, the provision of the required magnetic field may be easily accomplished by providing an equal number of turns per unit length along the length of the coil 31. Pipes 33 indicate diagrammatically means for introducing a propellent gas ysuch as, for example, lithium vapor, helium, or argon into the accelera-- tion space 13 intermediate the electrodes 21 and 23 and end Wall 26. Valve means (not shown) may be used' to control the rate at which the propellent gas is introduced into the acceleration space.

It is necessary that the supply of propellent gas be substantially uniform at the electrodes. Thus, any suitable means for introducing the propellent gas may be utilized and the distance from the point of injection of the propellent gas to the electrodes need be only sufficient to insure a supply of propellent gas at the electrodes. Electrodes 21 and 23 are connected to a source 34 of low voltage (S-500 volts) direct current through electrically conductive members 11 and 12 and conductors 35 and 36. The direct current source 34 permits the establishment of a direct current discharge, more thoroughly discussed hereinafter, indicated diagrammatically by broken lines 37 between electrodes 21 and 23 Referring now to a second embodiment of the present invention illustrated in FIGURE 3, the system may be essentially the lsame as the previous embodiment illustrated in FIGURE l. The cantilever inner member 11, the annular outer member 12, the coolant means 17 and 18, the electrode 21, the propulsion gas injection means 33, the direct current source 34 for the discharge 37, and the direct current source 32 for the magnet coil are also employed in the embodiment of FIGURE 3. However, the inner surface 24 of the outer annular member 12 and the outer surface 22 of the inner member 11 are exposed and not covered with insulation, electrode 23 has been omitted, and the means for supplying 'the magnetic field comprises two coils 41 and 42 to provide a decrease in field strength at the electrodes 21 and 23. The decrease in field strength is accomplished as shown in FIGURE 3 by providing a small air gap 43 concentric with electrode 21. Alternately, the number of coil windings can be decreased at the electrode 21 or a separate coil may be wound on the principal magnet coil to buck and hence decrease the strength of the magnetic field at electrode 21. This latter arrangement' is particularly attractive in that it permits infinite variation of the magnetic field `at the electrodes from the steady sta-te value to substantially Iany desired value. The variation of the field strength along the length of the acceleration space is shown in FIGURE 4. As shown in this figure, the solid line 51 indicates field strength along the length of the acceleration space and particularly for the embodiment illustrated in FIGURE 1, and the broken line 52 indicates the decrease in field strength at the electrode 21 for the embodiment illustrated in FIGURE 3. In contrast with the teaching of the prior art it is important to note that the field strength is substantially constant in the constant area portion of the acceleration space with the exception of the electrode region and that it decreases in the divergent portion of the acceleration space.

Referring now to a third embodiment of the present invention illustrated in FIGURE 7, the system may be essentially the same as the previous embodiment illustrated in FIGURE 1. Tlhe insulated annular outer member 12, the cooling means 17 and 18, the outer electrode 23, the propellent gas injecting means 33, the direct current source 34 for the discharge 37, and the direct current source 32 for the magnet coil 31 are lalso employed in the embodiment of FIGURE 7. However, in this embodiment the inner member 11, while a cantilever covered with insulating material 28, is terminated at about the upstream end 53 of the outer electrode 23. The unsupported end of the inner member 11 is tapered and coated with an emissive material to form the inner electrode 5S. As noted in connection with the discussion of the taper of inner member 11 in FIGURE l, the taper and point of divergence may also correspond to provide the aforementioned smooth transition. Additionally, a second magnet coil 56, surrounding the electrodes 27 and 55, is wound on the principal magnet coil 31. Coil 56 may be supplied with current from source 32 through a potentiometer 57 so that the current fiowing through this coil may be varied from, for example, zero to a preselected maximum amount. Further, the connection of coil 56 to source 32 is reversed from that of coil 31 to provide the aforementioned bucking magnet field. Alternately, the coil 56 may be wound in the direction opposite to that of the principal coil 31 and 6 the terminals connected to the source 32 in the same manner as that for the principal coil.

Referring now to va fourth embodiment of the present invention illustrated in FIGURE 8, the system may be essentially the same as the previous embodiment illustrated in FIGURE 7. However, in this embodiment the insulation on the inner and outer members 11 and 12 has been omitted, as has the second magnet coil. The electrode arrangement is identical to that illustrated in FIGURE 3, the tapered end portion of `the inner member 11 being extended slightly past the downstream end 53 of the outer electrode 23 -to permit the provision of the cylindrical inner electrode 21 oppositely disposed from and concentric with .the outer electrode 23. In this embodiment the propellent gas is introduced radially into the acceleration space through pipe 61 thus giving to the gas a spiral or swirling motion as it flows toward the electrodes 21 and 23.

In accordance with the present invention, the transport properties of a highly ionized plasma change when the electrons undergo one or more complete gyro-orbits between collisions, i. e., when wefe 1, Where we is the Y gyro frequency for the electrons and 're is the mean time between collisions. When were 1, the average distance the electrons diffuse across the magnetic field lines per collision is equal to the gyro radius instead of the mean free path, as is the case when were l. When we-re 1, the electron mobility perpendicular to the magnetic field lines is reduced. Thus, the ratio of the heat conduction due to electrons, with field, KB, to that without field, K, can be written:

For a low temperature accelerator equal ion and electon mobility is achieved when m1 1/4 wea-e: e

where m1 is the mass of the ions and me is the mass of the electrons. For most gases such as, for example, helium and lithium vapor ythis condition is realized .at wefelO, which is achieved by imposition of the axial magnetic field as shown and described hereinbefore. This permits control lof the transport of heat from the highly ionized plasma to the surfaces 22 and 24 of the device :which are lparallel t-o the magnetic field lines.

In addition, when wefe l and an electric field is applied normal to the field lines as by connection of supply 34 to members 11 and 12, the electrical conductivity oof the plasma is no longer a sealer quantity and Hall currents exist in the azimuthal direction. The electrical conductivity normal to the magnetic field lines is proportional to while the electrical conductivity along the field lines remains unchanged. The electron fiow which occurs perpendicular to the electric field and magnetic field direction (the aforementioned Hall currents) is thus a function of the strength of the magnetic field supplied by coil 31.

For the 'annular arc geometry shown and described herein, the effects of the magnetic field discussed above have a favorable effect on the arc operation which will now be described. As has been previously pointed out, when 1278 1, closed Hall currents are generated in the azimuthal direction, and since these currents are closed on themselves no Hall potential exists. One effect of this is that the ratio of loop or azimuthal current, i9, to radial or arc current fiow across the electrodes, jr, is equal to were. Of particular importance in this respect is the fact that the impedance of the arc is increased by the magnetic field since the radial conductivity is a function of were.

For the case where the plasma properties and current distribution are uniform across the channel, the conductivity of the plasma, a, is:

er o'o where ao is the plasma conductivity without a magnetic field. However, conductivity in the axial direction is not changed by the magnetic field, and the increased voltage in the radial direction due to this effect causes large axial currents in the vicinity of the electrodes. An analysis of this problem has shown that for the case of uniform plasma properties and electrodes which are short compared to the length of the channel, the increased axial currents change the impedance of the magnetic annular arc so that the effective conductivity of the plasma is inversely proportional to V (weTe)2 which would be the case if no axial currents were allowed to flow. Thus, for the geometry described the electrical impedance of the plasma is increased when were is greater than one, thereby providing an arc heating device wherein the voltage drop in the plasma is increased over that otherwise possible. This in turn provides an increase in efficiency since electrode losses are minimized.

Runs made at low pressures in helium with an arc discharge dissipative power of approximately 20 kw. produced a discharge voltage change from 47 volts without an axial magnetic field to S volts with an axial magnetic field of 4000 gauss, as shown by the curve of voltage vs. time in FIGURE 5. With this increase in voltage there was a corresponding decrease in current from 460 amperes to 220 amperes as shown by the curve of current vs. time in FIGURE 6. This change in discharge voltage and current indicates that the electrical resistance of the discharge increased from about 0.1 ohm to 0.63 ohm under conditions corresponding to a relatively constant total power input.

In actual practice the plasma is heated by the discharge between electrode 21 and electrode 23 and the plasma properties are not uniform throughout the channel. In particular, the incoming gas upstream of the discharge is at room temperature and does not carry current. The effect of the increased axial currents appears as an extension of the discharge currents downstream from the electrodes as illustrated, for example, in FIGURE 1. However, since large axial currents along with large azimuthal currents are not excluded, the arc impedance will show a dependence proportional to were instead of weZTeZ.

Total power, PT, delivered to the device can be written where PR is the power invested in creating rotational velocity and PD is the power invested in ionization and thermal motion. With the assumption that a complete recovery of thermal and azimuthal velocities is made, the final exhaust velocity can be written as where I is the specific impulse in seconds, vfiml is the final exhaust velocity, Cp is the specific heat at constant pressure, T is the temperature, and v0 is the azimuthal velocity of the plasma. At low values of I or low power, a device incorporating the present invention performs as a heater and the exit velocity is achieved by expanding the flow in portion 14 as in a conventional rocket where the thermal and rotational velocities are converted to directed velocities vx.

In the nozzle or divergent portion 14 the angular momentum of the plasma and the total energy are constant. Therefore, in the divergent nozzle the average radius of the spinning plasma increases as it moves toward the exit.

Since the angular momentum of the plasma is conserved the rotational velocity, v0, of the plasma decreases as it approaches the exit of the nozzle. Furthermore, since the total energy of the plasma is conserved the rotational kinetic energy of the plasma is transformed to axial kinetic energy as the plasma approaches the exit of the nozzle.

As the power level and particle energy is increased, the degree of ionization and the electrical conductivity of the plasma is also increased so that at high values of power delivered to the device most of the energy is invested in rotation of the plasma. This is true since at this level of power input the conductivity of the plasma is sufficiently high so that the rateof work done on the plasma with jr BX forces to achieve rotation exceeds the rate that energy is dissipated in the plasma due to Joule heating.

The axial velocity of the plasma vx at or downstream of the electrodes is of the order of the thermal velocity of the particles in the plasma and the azimuthal velocity va is independent of the axial velocity vx. Thus, kinetic energy may be added to the plasma in accordance with the present invention at a rate which is independent of the time required for the plasma to fiow through the device. No currents iiow in the divergent portion 14 which defines a nozzle. Where no currents flow, angular momentum is conserved. Thus, at the plasma passes through the divergent portion 14 its azimuthal velocity decreases, and as in an ordinary expansion nozzle, perpendicular energy is converted into directed energy in the exhaust direction.

In an embodiment of the present invention that was successfully operated with helium as the propellant the constant area annular geometry had an anode diameter of one and one-quarter inches, a cathode diameter of fiveeighths of an inch, and a length-to-diameter ratio of approximately 10. A thoriated tungsten electrode ring onehalf inch in length was used on the cathode which was water cooled. The anode consisted of a water cooled copper tube. The structure supporting the cathode also consisted of copper and was water cooled.

The arc dissipative power was supplied by a kw. direct current variable supply, and the direct current for the magnetic coil was supplied by a bank of batteries. The number density of the plasma was of the order of 1016/cm-3 to 1017/cm.3. The power levels of the arc were such that the average energy per particle was 2 to 3 electron volts. The strength of the axial magnetic field was from 4000 gauss to 10,000 gauss, which was large enough to make the gyro radius of the electrons smaller than their means free path.

The various features and advantages of the invention are thought to be clear from the foregoing description. Various other features and advantages not specifically enumerated will undoubtedly occur to those versed in the art, as likewise will many variations and modifications of the preferred embodiment illustrated, all of which may be achieved without departing from the spirit and scope of the invention as defined by the following claims.

I claim:

1. A plasma generator comprising: means defining a gas acceleration space closed at one end and divergent at its opposite end; an inner electrode intermediate the ends of said space; an outer electrode located within said space and encircling said inner electrode, said gas acceleration space having a substantially minimum transverse dimension at said electrodes, means for establishing a direct current discharge across said electrodes; means for establishing a magnetic field between said electrodes and through said space, to prevent the connection of said 'electrodes by magnetic lines of force; and means for introducing a gas into said space near its closed end.

2. A plasma generator comprising: means defining a gas acceleration space closed at one end and divergent at its opposite end; an inner electrode intermediate the ends of said space; an outer 'electrode located within said space and encircling said inner electrode, said gas acceleration space having a substantially minimum transverse dimension at said electrodes; means for establishing a direct current discharge across said electrodes; means for establishing a magnetic field within said space substantially parallel to its defining walls to prevent connection of said electrodes by magnetic lines of force; and means for introducing a gas into said space near its closed end.

3. A plasma generator comprising: means defining a gas acceleration space closed at one end and divergent at its opposite end; an inner electrode intermediate the ends of said space; an outer electrode located within said space and encircling said inner electrode, said gas acceleration space having a substantially minimum transverse dimension at said electrodes; means for establishing a direct current discharge across said electrodes; means for establshing a magnetic field within said space, said magnetic field being nonparallel with the defining walls of said space at said electrodes, substantially parallel with the remainder of said defining walls and preventing the connection of said electrodes by magnetic lines of force; and means for introducing a gas into said space near its closed end.

4. A pla-sma generator comprising: electrically conductive means defining a propellent acceleration space closed at one end and divergent at its opposite end; an electrically conductive member located within and intermediate the ends of said space, said propellent acceleration space having a substantially minimum transverse dimension at said electrically conductive member; means for applying a volttage across said electrically conductive means and said electrically conductive member; means for establishing a magnetic field within said space, said magnetic field being nonparallel with the defining walls of said space at said conductive member, substantially parallel with the remainder of said defining walls and preventing the connection of said electrically conductive members by magnetic lines lof force, said magneic field decreasing at said conductive member; and means for introducing a propellant into said space near its closed end.

5. A plasma generator comprising: electrically conductive first means defining an annular chamber closed at one end and divergent at the opposite end; electrically conductive second means coaxial with, encircled by, and insulated from said first means, said annular chamber having a substantially minimum transverse dimension at said electrically conductive second means; means for establishing a direct current discharge between said first and second means; means for establishing an axial magnetic field through the annulus defined by said first and second means, the lines of force of said magnetic field being nonparallel with the axial walls of said first means in a region intermediate the ends of said first means, substantially parallel with the remainder of said axial walls and preventing the connection of said first and second means by said magnetic lines of force; and means for introducing an ionizable gas into said chamber near its closed end.

6. The combination as defined in claim wherein the strength of said magnetic field is lower in said -region and said region is located intermediate the closed end of said chamber and its divergent end.

7. The combination as defined in claim 5 wherein said chamber is substantially cylindrical intermediate its closed end and its divergent end; said second means includes first annular electrode means located in the cylindrical portion of said chamber; said first means includes second annular electrode means coaxial with said first electrode means; and said discharge is established between said first and second electrode means.

8. A plasma generator comprising: electrically conductive first means defining an annular chamber comprising a cylindrical portion and a divergent portion, said chamber being closed at its end remote from the divergent portion; electrically conductive second means coaxial with, encircled by, and insulated from said first means, said annular chamber having a substantially minimum transverse dimension at -said electrically conductive second means; insulating material covering a majority of the inner surface of said first and the exposed surface of said sec-ond means, oppositely disposed annular regi-ons on said first and second means intermediate the ends of the cylindrical portion of said chamber being bare of said insulating material; means for establishing a direct current discharge between said annular regions; means for providing an axial magnetic field through said chamber, the lines of force of which are substantially parallel to the walls of said first means and do not connect said first and second means; and means for introducing an ionizable gas into said chamber near its closed end.

9. The combination as defined in claim 8 whe-rein said second means is located in the cylindrical portion of said chamber and the length of said chamber is considerably greater than the spacing between said first and second means.

10. The combination as defined in claim 8 wherein the strength of said magnetic field is lower at said annular region.

11. A plasma generator comprising: means defining an annular gas acceleration space closed at one end and divergent at its opposite end; a first electrode located within and intermediate the ends of said space; a second electrode located within said space and spaced from said first electrode, said gas acceleration space having a substantially minimum transverse dimension at said electrodes; means for applying a constant voltage across said electrodes to effect a discharge therebetween; means for introducing a gas into said space near its closed end; and means for establishing a magnetic field through said space and including said electrodes which does not connect said electrodes, thereby producing azimuthal movement of said gas through the interaction of the discharge currents and said magnetic field and closed Hall currents in the azimuthal direction.

12. A plasma generator comprising: means defining an evacuated annular gas acceleration space closed at one end and divergent at its opposite end; a first electrode located within and intermediate the ends of said space; a second electrode located within said space and spaced from said first electrode, said gas acceleration space having a substantially minimum transverse dimension at said electrodes; means for applying a constant voltage across said electrodes to effect a discharge therebetween; means for introducing a gas into said space near its closed end; and means for establishing a magnetic field between said electrodes and through said space producing a voltage drop in said gas intermediate said electrodes that is substantially greater than the voltage drop at said electrodes and preventing connection of said electrodes by magnetic lines of force.

References Cited bythe Examiner UNITED STATES PATENTS 2,052,796 9/36 Rava 313--23l.5 2,819,423 1/58 Clark 313-231 3,004,189 10/61 Giannini 315-111 3,007,072 10/61 MCGinn et al 60-355 3,041,824 7/62 Berhman 60-35.5 3,073,984 1/63 Eschenbach et al 313-63 FOREIGN PATENTS 20,697 1907 Great Britain.

SAMUEL LEVINE, Primary Examiner. JOHN W. HUCKERT, Examiner. 

1. A PLASMA GENERATOR COMPRISING: MEANS DEFINING A GAS ACCELERATION SPACE CLOSED AT ONE END AND DIVERGENT AT ITS OPPOSITE END; AN INNER ELECTRODE INTERMEDIATE THE ENDS OF SAID SAPCE; AN OUTER ELECTRODE LOCATED WITHIN SAID SPACE AND ENCIRCLING SAID INNER ELECTRODE, SAID GAS ACCESERATION SPACE HAVING A SUBSTANTIALY MINIMUM TRANSVERSE DIMENSION AT SAID ELECTRODES, MEANS FOR ESTABLISHING A DIRECT CURRENT DISCHARGE ACROSS SAID ELECTRODES; MEANS FOR ESTABLISHING A MAGNETIC FIELD BETWEEN SAID ELECTRODES AND THROUGH SAID SPACE, TO PREVENT THE CONNECTIN OF SAID ELECTRODES BY MAGNETIC LINES OF FORCE; AND MEANS FOR INTORDUCING A GAS INTO SAID SPACE NEAR ITS CLOSED END. 